The Quran

قيلَ لَهَا ادخُلِي الصَّرحَ ۖ فَلَمّا رَأَتهُ حَسِبَتهُ لُجَّةً وَكَشَفَت عَن ساقَيها ۚ قالَ إِنَّهُ صَرحٌ مُمَرَّدٌ مِن قَواريرَ ۗ قالَت رَبِّ إِنّي ظَلَمتُ نَفسي وَأَسلَمتُ مَعَ سُلَيمانَ لِلَّهِ رَبِّ العالَمينَ

Muhammad Asad

[After a while] she was told: "Enter this court!" - but when she saw it, she thought that it was a fathomless expanse of water, and she bared her legs.¹ Said he: "Behold, it is [but] a court smoothly paved with glass!"² Cried she: "O my Sustainer! I have been sinning against myself [by worshipping aught but Thee]: but [now] I have surrendered myself, with Solomon, unto the Sustainer of all the worlds!"

I.e., in order to wade into it, or perhaps to swim through it, thus braving the seemingly fathomless deep: possibly a symbolic indication of the fear which a human being may feel when his own search after truth forces him to abandon the warm, soothing security of his erstwhile social and mental environment, and to venture into the - as yet - unknown realm of the spirit.
I.e., not a dangerous, bottomless deep, as it appeared at first glance, but, rather, the firm, glass-clear light of truth: and with her perception of the ever-existing difference between appearance and reality, the Queen of Sheba comes to the end of her spiritual journey.